1. Field of the Invention
The present invention relates to an antifuse device having a uniform antifuse material thickness and a method for fabricating the same.
2. Description of the Related Art
Field programmable gate arrays (FPGAs) that can be programmed by users to implement virtually any set of functions sometimes employ antifuse devices for programmable elements. Antifuse devices generally include a pair of conductive electrodes separated by a dielectric layer. Amorphous silicon or α-silicon is frequently used for the dielectric layer, and thus prior to programming, the antifuse tends to exhibit very high resistance (typically >109 Ω) between the two electrodes. In a programming process, a predetermined voltage (the program voltage) is applied to the antifuse to break-down the dielectric material and create a low-impedance connection between the two conductive electrodes. Impedance of higher than 109 Ω may be reduced to about 20-50 Ω by the programming process.
The amorphous silicon based antifuse devices are easily programmed and exhibit great difference in impedance before and after programming, and are thus widely used in semiconductor devices such as programmable read only memory (PROM), programmable array logic (PAL), and FPGA devices.
Studies for reliability of programmed and unprogrammed α-silicon antifuses include: R. J. Wong and K. E. Gordon, “Evaluating the Reliability of the QuickLogic Antifuse”, Electronics Engineering, pp. 49-55 (June, 1992); and R. J. Wong and K. E. Gordon, “Reliability Mechanism of the Unprogrammed Amorphous Silicon Antifuse”, IEEE International Reliability Physics Proceedings, pp. 378-383 (1994). For integrity of low impedance of programmed antifuse, refer to: G Zhang, Y King, S. Eltoukhy, E. Handy, T. Jing, P. Yu, and C. Hu, “On-State Reliability of Amorphous Silicon Antifuses”, IEDM, pp. 551-554 (1995); or C. Shih et al., “Characterization and Modeling of a Highly Reliable Metal-to-Metal Antifuse for High-Performance and High-Density Field-Programmable Gate Array”, IEEE International Reliability Physics Proceedings, pp. 25-33 (1997).
In the antifuse device, the thickness of amorphous silicon is critical as the magnitude of program voltage needed to program the antifuse depends on it. In particular, the uniformity of the thickness of amorphous silicon has a significant effect on the reliability and yield of the programmable semiconductor devices. Generally, tens of millions of antifuses are fabricated in a single wafer. When amorphous silicon for the antifuse material is not deposited uniformly across the entire wafer, the program voltage may increase or decrease from place to place on the wafer. The variation in the antifuse material across the wafer is one reason for the variation in antifuse programming voltage. The range of programming voltages demonstrated by the antifuses is called the programming voltage distribution, which should be kept as tight or as small as possible for ensuring the reliability and yield of the semiconductor devices. Achieving a high uniformity of the antifuse material, amorphous silicon, is particularly difficult when it is deposited on an uneven surface such as in contact and/or via holes.